Female electrical contact assemblies find application in various types of electrical terminals that are used in various types of electrical devices, such as electrical receptacles. These contact assemblies typically include a plurality of individual contact pockets or chambers molded in the insulated body forming the electrical device housing. Mounted fixedly within each pocket is a female contact.
The female contact is typically constructed of a substantially flat base portion mounted in a rearward end of the contact pocket and two elongated, opposing contact arms cantilevered from the base portion. These contact arms extend forwardly toward a frontward opening in the pocket through which an elongated male blade contact member may be inserted. The contact arms are spaced apart at their forwardmost ends, which are located slightly rearwardly of the blade opening, to receive and retain the male blade contact longitudinally therebetween. The base portion is usually made part of an electrical terminal or a conductive member and the opposing spring contact arms are respectively inclined inwardly from the base member toward the longitudinal axis of the pocket, and then outwardly, to form a "knee" region near the free ends of the contact arms where the lateral spacing between the opposing arms is less than the width of the male contact blade. The arms extending forwardly from the knee region diverge or flare outwardly to provide mutually divergent forward tip surfaces for directing the tip of the male blade into the knee region and rearwardly thereof toward the base portion.
The male blade opening at the frontward end of the contact pocket is substantially centered on the longitudinal axis of the contact pocket and for various reasons known to those working in the art, conforms closely in both size and configuration to the size and cross-sectional configuration of the male contact blade. On the other hand, to provide the desired clearance for the outward displacement of the female contact arms, the cross-sectional area of the interior sidewalls of the contact pocket is necessarily considerably larger than the corresponding area of the male blade opening. Hence, the rearward opening to the wiring pocket may be molded much larger than the male blade opening thereby providing easier access for inserting a female contact into the pocket. Moreover, certain other conductive parts comprising the electrical device to which the female contact is mounted prior to assembly in the device housing such as the mounting yoke or conductive strap of an electrical receptacle, are typically located on the rearward surface of the electrical device adjacent the rearward end of the contact pocket. For ease of assembly, it is preferred that the female contact be placed in the housing by inserting it through an opening at the rearward end of the contact pocket. When properly mounted, the diverging female tips of each contact required by the receptacle are located slightly behind and in substantial longitudinal alignment with the corresponding male blade opening.
To provide good electrical contact with, and retention of, a male blade inserted into the female contact, the knee portions of the female contact are spaced slightly closer than the corresponding width of the male blade. With this arrangement, the free tips of the contact arms are displaced outwardly upon insertion of the male blade into the knee region.
The longitudinal movement of the male blade contact between the contact arms sometimes does not occur in the optimum longitudinal plane which passes symmetrically or midway between the opposite knee portions of the contact arms. Rather, the male blade may be inserted or withdrawn at a substantial angle with respect to this plane causing the male blade tip to bear against and drive portions of an opposing female contact blade arm outwardly toward, and in some extreme instances, against an opposing pocket sidewall. To lessen the possibility of this occurring, the portion of each contact arm rearwardly of the knee region is inclined outwardly to provide a greater lateral spacing between the contact arms in the vicinity of the contact base portion where contact with the blade tip is likely to occur.
Since the rearward end of the female contact arm is constrained against lateral movements by the fixed base portion from which it extends and a corresponding excessively deflected diverging tip will abut the opposing pocket sidewall, analyzed from a strength of materials standpoint, the contact arm is equivalent to a beam of rectangular cross-sectional shape fixed at one end (the base end), and supported at the other end (the tip end). Accordingly, depending upon such factors as the modulus of elasticity of the contact arm, the extent to which lateral displacement of a stressed portion of the arm is permitted by the contact pocket sidewall and the directions and magnitudes of the stresses produced in the arm by the male blade bearing thereagainst, the stresses developed in the contact arm may exceed the elastic yield point of the metal of which the arm is composed causing the highly stressed arm section to take a permanent set in its outwardly distorted position. In some cases, this can result in an increase in the lateral spacing between the contact arms in the knee region from the optimum spacing required for blade retention. Moreover, this condition can cause problems where good electrical continuity is a requirement for the optimum electrical performance of the contact assembly. The maintaining of proper contact pressures to ensure that electrical continuity exists at all times in the connection is extremely important in those cases where the male member is a grounding blade and the female contact is the grounding contact of an electrical wiring device.
The prior art has sought to prevent permanent distortion of the female contact by employing different techniques summarized briefly hereinbelow.
One prior art technique is to use special metal alloys in the contact arms, such as phosphor-bronze alloys, which possess substantially higher yield points than conventional brass alloy contact compositions so that the contact arms do not take a permanent set regardless of the degree to which they are distorted by the male blade. The disadvantage with this approach is that these higher yield point contacts are considerably more expensive that the conventional brass alloy compositions which otherwise possess all of the desired electrical conductivity properties for contact applications. Hence, a principal disadvantage in using the higher yield point alloys is that they add considerably to the cost of the device without appreciably improving the electrical performance of the contact assembly. In fact, in most cases, marginally lower conductivity is obtained with the special alloys. Also, as a practical matter, it presents a burden on the terminal device manufacturer to sample and qualitatively anaylyze the specially alloyed stock material to ensure that it has the desired alloy composition and deflection characteristics.
Another technique involves restraining the outward deflection of the contact arms by employing discrete deflection restraints attached to the arms during or after terminal assembly. Prior art restraints of this type include clip elements which clamp around the contact arms and extend from the knee region rearwardly to restrain the arms when they are displaced outwardly from one another more than a predetermined amount and unyielding ring elements which are placed over portions of the arms to inhibit the outward displacement thereof under the pressure of the male contact.
One disadvantage with the ring type of restraining devices is that they normally do not extend much beyond the knee region rearwardly of the contact arms and therefore, portions of the arms extending rearwardly of the knee region can still be distorted and overstressed by a tilted male blade tip bearing hard against those portions as it is inserted into or withdrawn from between the contact arms. While the clip elements do overlay rearward portions of the arms, these devices, as well as the ring devices, increase the cost of the device assembly and increase the inventory of parts required to be stocked by the terminal manufacture. Moreover, both of these types of devices create handling problems when they are fed into automatic assembling machines. Further, because they are not an integral part of the contact arms or the contact pocket, there is always the possibility that a device may be inadvertently omitted or lost in the assembly process. For these reasons, these restraining devices have not provided a completely satisfactory solution to the problem of preventing the contact arms from being overstressed.
A third technique involves restraining the outward deflection of the contact arms by the use of a resilient restraining device mounted on one or more sidewalls of the wiring pocket. Prior art devices of this type include resilient bumper pads or coil springs interposed between a pocket sidewall and an opposing portion of a contact arm. In general, these devices have the same disadvantages as the restraining type of device which is placed over one or both of the contact arms and therefore, have also not proven to be a satisfactory solution to the problem of preventing overstressing of the contact arms.
Another prior art technique is to use supplementary leaf springs mounted so as to provide additional strength to the contact arms to keep the stresses below the yield point of the contact material. Usage of these devices, however, also adds to the cost of the device and the installation and proper performances of such springs is not easy for the device manufacturer to provide and ensure.